Current mirrors are utilized in a myriad in applications. Further, it is desirable to provide a current mirror that can be enabled (and thus disabled) especially when designing integrated circuits for portable products where power dissipation is critical. That is, by disabling a current mirror, substantial current drain can be eliminated when operating in a standby mode.
Typically, a current mirror consists of an input transistor and an output transistor wherein the bases of each are coupled together and the emitters of each are coupled to a first supply voltage terminal. One attempt for disabling a current mirror includes coupling the current carrying electrodes of an enable transistor between the collector of the input transistor of the current mirror and a second supply voltage terminal. Further, the base of the enable transistor is coupled to receive an enable signal. In general, the enable signal provides current to the current mirror through an emitter follower buffer (the enable transistor). However, this circuit suffers from two problems. First, the current in the current mirror is directly dependent upon the voltage of the enable signal. Second, the voltage appearing at the input of the current mirror is reduced by one diode drop due to the enable transistor. This will increase the temperature dependence of the current through the current mirror and will increase the minimum supply voltage that is required to operate the current mirror.
Another attempt at disabling a current mirror involves coupling the current carrying electrodes of an enable transistor between the common bases of the current mirror transistors and the first supply voltage terminal. Further, the base of the enable transistor is coupled to receive an enable signal. In general, the enable signal is used to pull the base voltage of the input transistor below its turn on voltage thereby disabling the current mirror. However, this circuit has the disadvantage of drawing substantial current even during standby mode.
Hence, there exists a need for an improve current mirror that can be disabled while minimizing the power dissipation.